Most of the image-forming apparatuses, such as laser toner printer or fax machine, require a high-voltage generator to provide the required voltage for operation. The high-voltage generator employs a single-switch resonant circuit and enables the transistor to work in the active region, so that a low voltage received by the primary side of the transformer can be converted into a sinusoidal-wave voltage whose amplitude can be regulated by a voltage feedback signal. The sinusoidal-wave voltage is amplified across the secondary side of the transformer and converted by a rectifier/multiplier unit into a high-level DC voltage whose voltage level can be regulated for the image-forming apparatus.
FIG. 1 shows a circuit diagram of a high-voltage generator for laser toner printer according to the prior art. As shown in FIG. 1, a high-voltage generator includes a high-voltage transformer T12 having a primary coil N11, a secondary coil N12 and an auxiliary coil N13. A pulse-width-modulation (PWM) signal 41 is filtered by a filter consisted of a resistor R11 and a capacitor C11, thereby generating a DC reference voltage Vref at the positive input terminal of an operational amplifier 18, in which the DC reference voltage Vref can be adjusted depending on the duty cycle of the PWM signal 41. One end of the primary coil N11 is connected to an input DC voltage Vin, and the other end of the primary coil N11 is connected to the collector of a switch device 16 which is implemented by a NPN-type bipolar junction transistor. The base of the switch device 16 is connected to one end of the auxiliary coil N13, and the emitter of the switch device 16 is connected to ground. A feedback control unit 11 is connected between the output of the high-voltage generator and the operational amplifier 18 for detecting the variations on the output voltage of the high-voltage generator and in response to the results of detection sending a feedback signal to the negative input terminal of the operational amplifier 18. The feedback signal is compared with the DC reference voltage Vref and in response to the results of comparison generates a voltage control signal 48. The voltage control signal 48 is sent to the other end of the auxiliary coil N13 for controlling the base DC current of the switch device 16; and the AC current on the base of the switch device 16 is provided through the auxiliary coil N13 whose voltage is induced by the primary coil N11, thereby reaching the goal of changing the oscillating amplitude of the oscillating voltage on the primary coil N11. Therefore, the voltage control signal 48 can control the oscillating amplitude of the oscillating voltage on the primary coil N11, thereby regulating the secondary output Vout of the high-voltage generator to be the voltage specified by the duty cycle of the PWM signal 41. Here, the oscillating voltage on the primary coil N11 is amplified by the secondary coil N12 and thus a high-level AC voltage is induced across the secondary coil N12. Next, the high-level AC voltage is converted by a secondary rectifier/multiplier unit 13 into a high-level DC output voltage Vout which is a positive voltage or a negative voltage having a voltage level of thousands or hundreds volts. The polarity of the high-level output voltage Vout depends on the connection topology of the feedback control unit 11 and the secondary rectifier/multiplier unit 13. The scheme that presents a similar topology with the high-voltage generator of FIG. 1 is given in U.S. Pat. No. 6,051,921 and US Patent Publication No. 2006/0092672, all of which are incorporated herein for reference.
Although the high-voltage generator of FIG. 1 can provide a sufficient high-level output voltage Vout to drive the image duplicating elements within an image-forming apparatus, some crucial drawbacks are still unresolved. The most significant drawback is caused by the feedback signal provided to the negative input terminal of the operational amplifier 18 which is generated by downscaling the secondary output of the high-voltage generator through the feedback control unit 11. As state above, the secondary output Vout of the high-voltage generator is a high-level DC voltage having a voltage level of thousands volts. Nonetheless, the voltage level of the feedback signal which is sent to the negative input terminal of the operational amplifier 18 is generally several volts. Therefore, the feedback control unit 11 must include a voltage divider consisted of high-impedance resistors. In this case, the output transient response of the high-voltage generator to the variations on the PWM signals or the output impedance will become very slow. If it is desired to improve the output transient response, the impedance of the voltage-dividing resistors within the feedback control unit 11 has to be reduced. However, this would cause a considerable power loss. In addition, if it is desired to change the polarity of the output voltage Vout, the polarity of the diodes within the secondary rectifier/multiplier unit 13 has to be reversed and the connection topology of the feedback control unit 11 has to be changed as well. Therefore, the scheme set forth in FIG. 1 is not possible to satisfy the needs of providing outputs with different polarities by the same printed circuit board.
FIG. 2 shows a modified high-voltage generator according to the prior art. As shown in FIG. 2, a high-voltage transformer T22 has a primary coil N21, a secondary coil N22, an auxiliary coil N23 and a voltage detection coil N24. An input DC voltage Vin is connected to one end of the primary coil N21, and the collector of a switch device 26 which is implemented by a NPN-type bipolar junction transistor is connected to the other end of the primary coil N21. The base of the switch device 26 is connected to one end of the auxiliary coil N23, and the emitter of the switch device 26 is connected to a ground terminal. The voltage detection coil N24 is configured to generate an AC voltage associated with the secondary output Vout of the high-voltage generator. The AC voltage outputted from the voltage detection coil N24 is converted by a primary rectifier/multiplier unit 25 into a voltage detection signal 42, which represents the secondary output Vout of the high-voltage generator. Next, the voltage detection signal 42 is divided by a voltage divider (R21, R22) into a fractional voltage detection signal 43, which is inputted into a voltage control unit 27 along with a pulse-width modulation signal 44. The voltage control unit 27 compares the fractional voltage detection signal 43 with a DC voltage which is generated by filtering the pulse-width modulation signal 44 through a filter consisted of a resistor R25 and a capacitor C25, and in response to the results of comparison outputs a voltage control signal 49. The voltage control signal 49 is sent to the other end of the auxiliary coil N23 through resistors R23 and R24, and provided to the base of the switch device 26 through the auxiliary coil N23. The voltage control signal 49 is configured to control the base current of the switch device 26 and thus control the oscillating amplitude of the oscillating voltage on the primary coil N21, thereby generating the secondary output Vout of the high-voltage generator to the voltage specified by the duty cycle of the pulse-width modulation signal 44. Here, the voltage on the primary coil N21 is amplified by the secondary coil N22, and the amplified voltage is induced across the secondary coil N22. The high-level AC voltage is converted by the secondary rectifier/multiplier unit 28 into a high-level DC output voltage Vout which is a positive voltage or a negative voltage having a voltage level of thousands or hundreds volts. It is to be noted that the polarity of the high-level output voltage Vout does not depend on the primary rectifier/multiplier 25 or the voltage control unit 27 of FIG. 2 but depends on the connection topology of the secondary rectifier/multiplier unit 28 only. Because the fractional voltage detection signal 43 is generated by low-impedance voltage-dividing resistors, the output transient response of the high-voltage generator can be improved. More advantageously, regardless of the polarity of the secondary output Vout, the voltage detection coil N24 and the primary rectifier/multiplier unit 25 can generate a voltage detection signal which is a positive voltage. Therefore, if it is desired to change the polarity of the secondary output Vout, it can be accomplished by reversing the polarities of the rectifying diodes within the secondary rectifier/multiplier unit 28 without modifying the circuitries of the primary rectifier/multiplier 25 and the voltage control unit 27. Therefore, it is possible to provide outputs with different polarities by the same printed circuit board. The scheme that presents a similar topology with the high-voltage generator of FIG. 2 is given in U.S. Pat. No. 6,529,388, which is incorporated herein for reference.
It can be understood from the above descriptions that the high-voltage generator as shown in FIG. 2 is configured to generate an AC voltage associated with the secondary output Vout of the high-voltage generator across the voltage detection coil N24, which is located in the low-voltage side of the transformer, so that the resistance of the resistor R21 does not have to be large to obtain the desired detection signal 43. Also, the AC voltage associated with the secondary output Vout of the high-voltage generator is converted by the primary rectifier/multiplier unit 25 into a voltage detection signal indicative of the secondary output Vout. Therefore, the output transient response of the high-voltage generator to the variations on the PWM signal or the output impedance can be improved. Furthermore, regardless of the polarity of the secondary output Vout (which depends on the polarities of the rectifying diodes within the secondary rectifier/multiplier 28), the voltage level of the voltage detection signal 42 is absolutely positive. Therefore, one printed circuit board can provide outputs with different polarities. Nonetheless, the high-voltage generator of FIG. 2 requires an additional voltage detection coil N24 to generate the voltage detection signal 42, and thus the manufacturing cost of the high-voltage transformer T22 is increased and the efficiency of the high-voltage transformer T22 is deteriorated.
There is a tendency to develop a high-voltage generator that can achieve output voltage regulation with a minimum number of transformer coils and improve the output transient response and reduce the power loss, and further enhance the compatibility of the printed circuit board for outputs with different polarities.